How Can I Get Phpmailer Mailings to Send Message With a return-path to Another Domain

I'm using a 3rd party SMTP service for sending my newsletters. Because of that, my ISP does not accept bounces because they are coming from an email not originating with them. Okay. So I set up a mailbox with my SMTP service to accept the bounces.

However, my mailing list program is refusing to send out emails whose return-path has a different domain than the from field.

I believe this is caused by phpmailer in it's mailsend routine:

The key code appears to be this, but I'm not that much of an expert with PHP to figure out how to get around whatever check it is doing, which I think has something to do with that safe_mode. The return-path value that I want to use is in the variable: $this->Sender

  /** 
   * Sends mail using the PHP mail() function. 
   * @param string $header The message headers 
   * @param string $body The message body 
   * @access protected 
   * @return bool 
   */ 
  protected function MailSend($header, $body) { 
    $toArr = array(); 
    foreach($this->to as $t) { 
      $toArr[] = $this->AddrFormat($t); 
    } 
    $to = implode(', ', $toArr); 
    $params = sprintf("-oi -f %s", $this->Sender); 
    if ($this->Sender != '' && strlen(ini_get('safe_mode'))< 1) { 
      $old_from = ini_get('sendmail_from'); 
      ini_set('sendmail_from', $this->Sender); 
      if ($this->SingleTo === true && count($toArr) > 1) { 
        foreach ($toArr as $key => $val) { 
          $rt = @mail($val, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header, $params);
           // implement call back function if it exists 
          $isSent = ($rt == 1) ? 1 : 0; 
          $this->doCallback($isSent,$val,$this->cc,$this->bcc,$this->Subject,$ body);
         } 
      } else { 
        $rt = @mail($to, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header, $params);
         // implement call back function if it exists 
        $isSent = ($rt == 1) ? 1 : 0; 
        $this->doCallback($isSent,$to,$this->cc,$this->bcc,$this->Subject,$b ody);
       } 
    } else { 
      if ($this->SingleTo === true && count($toArr) > 1) { 
        foreach ($toArr as $key => $val) { 
          $rt = @mail($val, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header, $params);
           // implement call back function if it exists 
          $isSent = ($rt == 1) ? 1 : 0; 
          $this->doCallback($isSent,$val,$this->cc,$this->bcc,$this->Subject,$ body);
         } 
      } else { 
        $rt = @mail($to, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header);
         // implement call back function if it exists 
        $isSent = ($rt == 1) ? 1 : 0; 
        $this->doCallback($isSent,$to,$this->cc,$this->bcc,$this->Subject,$b ody);
       } 
    } 
    if (isset($old_from)) { 
      ini_set('sendmail_from', $old_from); 
    } 
    if(!$rt) { 
      throw new phpmailerException($this->Lang('instantiate'), self::STOP_CRITICAL);
     } 
    return true; 
  }

Does anyone know what in this code is preventing me from using a different domain for my return-path, or better yet, does anyone know how I can fix (or hack) this so it will send out my mail?

@Sanmai's comment got me looking at the parameters. When I started testing some of them in the phpmailer routine, I found the code wasn't executed. So at least he helped me realize the problem's somewhere else.

I still have the problem. I'll now try to better isolate it. Then maybe I can solve it, and if not, I'll modify this question and try again.

Thanks for giving me a bit of something to go on.